The other day I had the following problem: I had a list of Transfoms and I wanted to remove the entries that met a certain condition. This is what I had written:
foreach(Transform trns in transformList){
if(AlreadyAligned(trns))
transformList.Remove(trns);
}
This gave me the following error message: InvalidOperationException: Collection was modified; enumeration operation may not execute.
This makes sense, I'm modifying the list I'm currently reading and there is no way I can be sure I'll catch all entries. There are three ways around this problem: The first is to use LINQ (C# only), the second is instead of removing the entries at once to store them in a separate list and then use that list in a second foreach-loop to remove the entries from the first list and the third solution is to use a for-loop. After some reseach I reached the conclusion that the for-loop is the fasted way to achieve the needed results. Here is the code:
int counter = transformList.Count;
for(int i = 0; i <= counter - 1; i++){
if(AlreadyAligned(transformList[i])){
transformList.RemoveAt(i);
i --; //reduce the indexer because we removed an entry from list
counter --; //reduce the counter since the list has become smaller
}
}
First of all we need to know how many entries the list has initially. I could have used transformList.Count in the loop directly but this way I don't have to call the Count method for each iteration. Then we set up the loop, we start at index 0 and go to counter - 1 (because the index starts at 0 instead of 1). We perform our check and if it succeeds we remove the entry with index i. Since the list has now been shortened we need to reduce i because an item that has been at a position of 7 for example is now at position 6 (assuming that i is less or equal to 6 in this case). We also need to reduce our counter because the length of the list has been shortened as well.
As you can see the amount of lines has been doubled, while using LINQ would have left the code short. If you know that your list will always be small enough I'd go for the slower but shorter approach. In my case though I have no idea how large the list will eventually be, so using the for-loop makes perfect sense. I hope some of you find this useful in the future.
